The present invention relates generally to process monitors for evaluating the relative strength of a semiconductor device fabrication process, and more particularly, to a new process monitor which utilizes an existing impedance controller on the semiconductor device as the process monitor.
Monitoring silicon fabrication process statistics is very important for semiconductor manufacturers to ensure the quality of the integrated chip or semiconductor devices produced and to provide better control of the semiconductor fabrication processes used. Process deviations during fabrication result in the production of semiconductor devices which exhibit a wide range of characteristics. If a particular device exhibits "strong" characteristics (such as low output impedances, high drive currents and faster speed), the process used to fabricate the particular semiconductor device will be referred to as a "strong process". On the other hand, devices exhibiting "weak" characteristics (such as high output impedances, low drive currents and slower speeds) are frequently described as being fabricated using a "weak process".
Process monitor circuitry is used by manufacturers of semiconductor devices to help identify the relative strengths and weaknesses of the devices produced, and thus of the processes used to fabricate the devices. Conventionally, dedicated process monitor circuitry is included on the silicon used to fabricate the semiconductor device, and includes a long chain of linked delay elements. The delay of an electrical signal across the dedicated process monitor circuitry is measured and compared with the results provided by other process monitors from other semiconductor devices, or with data from computer simulations. Based upon the results of the comparison, the manufacturer can determine whether the process used to manufacture the particular semiconductor device was fast or slow as compared to the nominal process results.
Conventional process monitors and methods of using the same introduce a number of problems and disadvantages. For example, conventional process monitor circuitry is used solely for the purpose of monitoring the strength of the device and of the process. Since there are no other intended uses for the circuitry, inclusion of the process monitor on the device is inefficient use of silicon area. Further, to increase measurability and to guarantee proper correlation with the performance of real circuits, a large number of delay elements must typically be included in the process monitor circuitry in order to obtain at least some minimum delay. As process advances are made, the delay introduced by each delay element has been reduced such that even longer chains of delay elements have to be used to obtain the minimum necessary delay. This further increases the silicon area required for the process monitor. In general, an undesirable trend has arisen: the faster the process becomes, the bigger the process monitor needed.
Therefore, there is a need for an improved process monitor which overcomes the deficiencies and limitations of conventional process monitors and methods of using the same.